Almost a quarter of people who drink bottled water at home say they do so because they believe it is 'better for them' than tap water, according to market researchers Mintel.

But what these consumers may not realise is that tap water must be checked daily under a rigorous inspection regime, researchers said.

It also contains trace amounts of chlorine that prevent the spread of anything harmful such as bacterial infections. By contrast, makers of bottled water are only required to undertake monthly testing at source. Once filled and sealed, a bottle of water might remain in storage for months before it is sold.

Bottled water contains no disinfecting additives such as chlorine.

After a bottle of water is opened it has no way of remaining sterile, and so must be drunk within days. "Water coming from UK taps is the most stringently tested in the world. People think there must be something wrong with tap water because it is so cheap and plentiful. But from a safety and price perspective, tap water is better for you," Professor Paul Younger of Glasgow University said.

"If the bottle is accidentally opened or someone tampers with it, then it can easily get contaminated," added Younger, author of 'Water: All That Matters'. "There's certainly a greater chance you could find something harmful in bottled water than from your taps. Ideally it should be drunk on the day it is opened, as it can easily pick up bacteria from someone's hands or face," Younger said.

Sue Pennison of the Drinking Water Inspectorate, which audits household supplies, said out of more than four million samples of tap water last year, 99.96 per cent passed strict standards. "Tap water is safe to drink, everything else is a personal lifestyle choice," she said. Jo Jacobius, director of British Bottled Water Producers, said all water available in the UK is "highly regulated and generally of good quality". She added that most bottled water companies test on a daily basis.

Natural bottled mineral water must come from an officially recognised underground spring, be bottled at source and cannot be treated or filtered.